A GREAT PERMIAN DELTA 559 



little more than tantalize the student with the hints of new forms and 

 new relationships which can not be verified. In some places these water- 

 worn fragments are so thick upon the ground that they can literally 

 be shoveled up by the wagon load. In some rare instances the bodies 

 of animals found their way unharmed into the water and, distended 

 by gases of decomposition, floated far and uninjured until they came 

 to rest on some mud flat beyond the reach of sharks or other predatory 

 animals. Such skeletons are preserved entire and in a most wonderful 

 state of preservation, but they are exceedingly rare, not more than one 

 turning up in a season's search. 



Of all the wonderful animals revealed by their petrified remains, 

 perhaps the most striking are the reptiles. The reptiles made their 

 first appearance in the Permian or in the latter portion of the Car- 

 boniferous preceding, but here at the very inception of their line they 

 developed a great diversity of form and habit. There were aquatic and 

 terrestrial forms, carnivorous, herbivorous and omnivorous forms, forms 

 simple and closely resembling their amphibian ancestors and forms so 

 bizarre in their structure that the world has produced nothing more 

 strange. The reptiles descended from the amphibians and it is natural 

 to look in these beds, where the lowest of the reptiles are found, for 

 the connecting link between the two, but as yet this form has not been 

 discovered; the approach from both sides, however, is so close that it 

 is frequently impossible to determine the nature of a specimen from 

 a single bone or small portion of the skeleton. 



The simplest of the early carnivorous reptiles were aquatic, living 

 in the waters of the great rivers or perhaps even in the ocean. The 

 body was long and slender and" the tail was exceptionally so, in corre- 

 lation with the swimming habit. Aside from the more technical points, 

 the interest in the development of the primitive reptiles centers in 

 certain changes of the teeth and the dorsal spines of the vertebra?. In 

 one of the simplest forms, Poliosaurus, the teeth have the form of sim- 

 ple cones of nearly equal size in all parts of the jaw; such a dentition 

 indicates that it preyed upon small animals which it seized and swal- 

 lowed whole after the manner of snakes and many lizards. The dorsal 

 spines are low and do not project beyond the skin. The animal prob- 

 ably resembled very closely the living monitor of the Nile. 



In another and closely related reptile, Theroplcura, the teeth have 

 become differentiated, those at the anterior end of both the upper and 

 the lower jaws, the incisors, are enlarged and have taken on the ap- 

 pearance of tusks; posterior to the incisors there is a slight notch in 

 the edge of the upper jaw, caiised by the growth of the lower incisors, 

 and posterior to this there are tusk-like teeth in the upper jaw, the 

 canines. The growth of the incisors and canines increased the power 

 of grasping and holding prey. Moreover, in the posterior teeth of the 



